This invention relates to an amplifier failure detection apparatus, and more particularly to an amplifier failure detection apparatus of a radio transmitter that comprises a function for correcting distortion of the amplifier of the radio transmitter, and detects the gain of the amplifier drops a set level or more in order to determine that amplifier failure has occurred.
Recently, high-performance transmission through digitization is often used in radio transmission. When multi value phase modulation is used in radio transmission, techniques for linearizing the amplification characteristics of a power amplifier, suppressing non-linear distortion, and reducing the power leakage to adjacent channels is particularly important; also when using an amplifier having poor linearity to improve power efficiency, it is essential that a technique be used to compensate for the resulting distortion.
FIG. 9 is a block diagram showing an example of a transmission apparatus in a conventional radio transmitter, where a transmission-signal-generation device 1 transmits a serial digital data sequence, and a serial-to-parallel converter (S/P converter) alternately divides the digital data sequence one bit at a time into two sequences, an In-phase component signal (I signal) and a Quadradture component signal (Q signal). A digital-to-analog converter 3 converts both the I signal and Q signal to analog baseband signals, and inputs them to a quadrature modulator 4. The quadrature modulator 4 multiplies the input I signal and Q signal (transmission baseband signals) by a reference carrier signal and a signal whose phase is shifted 90° from the reference carrier signal, respectively, and by adding the multiplication results, performs quadrature modulation and outputs the result. A frequency converter 5 performs frequency conversion by mixing the quadrature-modulated signal and a local oscillation signal, and a power amplifier (PA) 6 amplifies the power of the carrier signal that is output from the frequency converter 5 and that signal is then radiated into air from an aerial wire (antenna) 7.
In mobile communication such as W-CDMA, the transmission power of the transmission apparatus is a large 10 W to several 10 W, and the input/output characteristics (distortion function f(p)) of the power amplifier 6 becomes non-linear as shown by the dotted line in (a) of FIG. 10. Non-linear distortion occurs due to this non-linear characteristic, and in the frequency spectrum near the transmission frequency f0, a side robe emerges as shown by the solid line in (b) of FIG. 10, thus power leaks to the adjacent channels causing adjacent interference. In other words, due to the non-linear distortion, the power of the transmission wave that leaks into the adjacent frequency channels becomes large as shown in (b) of FIG. 10. This power leakage becomes noise in other channels, which causes the quality of communication in those channels to become poor. Therefore, this is strictly regulated.
For example, the power leakage is small in the linear range of the power amplifier (see (a) of FIG. 10) and becomes large in the non-linear range. Therefore, so as to make the power amplifier a high-output amplifier, it is necessary that the linear range be increased. However, in order to increase the linear range, an amplifier having performance which is greater than that actually required becomes necessary, and thus there is a problem in that cost and size of the apparatus become a disadvantage. Therefore, a transmission apparatus having a distortion-compensation function that compensates for the distortion that causes the non-linearity of the power amplifier is used.
FIG. 11 is a block diagram showing a transmission apparatus having a digital-non-linear-distortion-compensation function that uses a Digital Signal Processor (DSP). A digital data group (transmission signal) that is transmitted from a transmission-signal-generation device 1 is converted to two sequences, an I signal and Q signal, by a S/P converter 2, and then input to a distortion-compensation unit 8 that comprises a DSP. The distortion-compensation unit 8 comprises: a distortion-compensation-coefficient-memory unit 8a that stores a distortion-compensation coefficient h(pi) that correspond to the power level pi (i=0 to 1023) of the transmission signal x(t); a predistortion unit 8b that performs distortion-compensation processing (predistortion) on the transmission signal; and a distortion-compensation-coefficient-calculation unit 8c that compares the transmission signal x(t) with the demodulated signal (feedback signal) that is demodulated by a quadrature demodulator that will be described later, then calculates and updates the distortion coefficient h(pi).
The distortion-compensation unit 8 performs predistortion processing on the transmission signal, after which it inputs the transmission signal to a DA converter 3. The DA converter 3 converts the input I signal and Q signal to analog baseband signals and inputs the results to a quadrature modulator 4. The quadrature modulator 4 performs quadrature modulation by multiplying the input I signal and Q signal by a reference carrier wave and a signal whose phase is shifted 90° from that reference carrier wave, respectively, then adding the multiplication results and outputting the results. A frequency converter 5 mixes the quadrature-modulated signal and a local oscillation signal and performs frequency conversion, after which a power amplifier 6 amplifies the power of the carrier-wave signal that is output from the frequency converter 5, and radiates that signal from an aerial wire (antenna) 7.
Part of the transmission signal is input to a frequency converter 10 via a directional coupler 9, where the frequency converter 10 down-converts the frequency and then inputs the signal to a quadrature-demodulator 11. The quadrature demodulator multiplies the input signal by both a reference-carrier wave and a signal whose phase is shifted 90° from that carrier wave, and performs quadrature demodulation to restore the I and Q baseband signals of the transmission side, and inputs the results to an AD converter 12. The AD converter 12 converts the input I and Q signals to digital signals and inputs them to a distortion-compensation unit 8. The distortion-compensation unit 8 performs adaptive signal processing that uses a LMS (Least Mean Square) algorithm to compare the transmission signal before distortion compensation with a feedback signal that was demodulated by the quadrature demodulator 11, and calculates and updates the distortion-compensation coefficient h(pi) so that the difference between the two signals becomes zero. After that, by repeating the operation described above, the non-linear distortion of the power amplifier 6 is suppressed, and the power leakage to adjacent channels is reduced.
FIG. 12 is a drawing showing the distortion-compensation process by adaptive LMS, and omits the modulation/demodulation unit, frequency conversion unit, etc. In FIG. 12, 15a is a multiplier (corresponds to the predistortion unit 8b in FIG. 11) that multiplies the transmission signal x(t) by a distortion-compensation coefficient hn(p), 15b is a DA converter that converts the distortion compensated signal to an analog signal, 6 is a power amplifier (PA) that has distortion characteristics represented by a distortion function f(p), 15d is a feedback system that feeds back the output signal y(t) from the power amplifier, 15e is an AD converter that converts the feedback signal to a digital signal, 15f is a power-calculation unit that calculates the power p(=|x(t)|2) of the transmission signal x(t) and outputs that power p as the read address of the distortion-compensation-coefficient-memory unit, 15g is a distortion-compensation-coefficient-memory unit (corresponds to the distortion-compensation-coefficient-memory unit 8a in FIG. 11) that stores a distortion-compensation coefficient that corresponds to the power of the transmission signal x(t), and together with outputting a distortion-compensation coefficient hn(p) that corresponds to the power p of the transmission signal x(t), updates the old distortion-compensation coefficient hn(p) with the distort ion-compensation coefficient hn+1(p) that is set according to the LMS algorithm.
Also, in FIG. 12, 15h is a distortion-compensation-coefficient-calculation unit that calculates a distortion-compensation coefficient hn+1(p) according to a LMS algorithm, 15i is a delay circuit for generating an address in the distortion-compensation-coefficient-memory unit 15g for writing the distortion-compensation coefficient hn+1(p), and this delay circuit 15i and the power-calculation unit 15f constitute an address-generation unit 15j. Moreover, 15k and 15m are delay circuits that adjust the timing of the transmission signal x(t) and feedback signal y(t), and control the delay time of each signal so that both the transmission signal x(t) and feedback signal y(t) are input simultaneously to the distortion-compensation-coefficient-calculation unit 21.
In the distortion-compensation-coefficient-calculation unit 15h, 21 is a subtractor that outputs the difference e(t) between the transmission signal x(t) before distortion compensation and the feedback signal y(t), 22 is an operational circuit that comprises: a multiplier 22a that multiplies the error e(t) by a step-size parameter μ; a complex-conjugate-signal-output unit 22b that outputs a complex-conjugate signal y‡(t); a delay circuit 22c that adjusts the timing that the distortion-compensation coefficient hn(p) is output; a multiplier 22d that multiplies hn(p) and y‡(t); a multiplier 22e that multiplies μe(t) and u‡(t); and an adder 22f that adds the distortion-compensation coefficient hn(p) and μe(t) u‡(t). With the construction described above, the calculation shown below is performed.hn+1(p)=hn(p)+μe(t)u‡(t)e(t)=x(t)−y(t)y(t)=hn(p)x(t)f(p)u(t)=x(t)f(p)=hn(p)‡y(t)p=|x(t)|2 
Here, x, y, f, h, u and e are complex numbers, and ‡ is the complex conjugate. By performing the calculation above, the distortion-compensation coefficient h(p) is updated so that the difference signal e(t) between the transmission signal x(t) and feedback signal y(t) becomes a minimum, and finally, this converges to the optimum value of the distortion-compensation coefficient, and the distortion of the power amplifier 6 is compensated.
A radio transmitter having the distortion-compensation function described above is used in a base station apparatus in a mobile radio system. When a base station apparatus breaks down, it has a large effect on the users, so it is necessary that communication continues with no breaks, and in order to do this, countermeasures such as redundant construction of the power amplifier are being taken. In order to cope with that construction, it is necessary that amplifier failure be accurately and quickly detected, and then the amplifier be switched, or if necessary, to quickly notify the administrator of amplifier failure. In order to do this, the radio transmitter comprises a function for detecting that the gain of the power amplifier drops a set level or more, and thereby determining that failure of that power amplifier has occurred.
FIG. 13 is a drawing showing the construction of a transmission apparatus in a radio transmitter having an amplifier failure detection unit, where the same reference numbers are given to parts that are the same as those in FIG. 11 and FIG. 12. The quadrature modulator 4 and frequency converter 5 in FIG. 11 are shown as a modulator/frequency converter (MDFU) 31, and the frequency converter 10 and quadrature demodulator 11 in FIG. 11 are shown as a demodulator/frequency converter (DMFD) 32.
An amplifier failure detection unit 33 detects the gain of the power amplifier 6, and it comprises: a gain-detection unit 33a that outputs a voltage signal (gain-monitor voltage) Vout that corresponds to the gain; an alarm-threshold-value-generation unit 33b that generates a fixed alarm threshold level VAL, and a comparator 33c that compares the gain that is detected by the gain-detection unit 33a and the alarm-threshold level, and generates an alarm based on the comparison results. The gain-detection unit 33a is built inside the power amplifier 6, however, in the drawing it is shown on the outside of the power amplifier 6.
FIG. 14 is a drawing showing the construction of the power amplifier 6 having a gain-detection function, and comprising: an amplifier 6a; a directional coupler 6b that extracts part of the input signal from the input side of the amplifier 6a; a directional coupler 6c that extracts part of the output signal from the output side of the amplifier 6a; an attenuator 6d; and a gain detector 33a that is created with a gain-detection IC circuit.
The gain-detection unit 33a comprises two log-amp detectors 33a-1, 33a-2 and a computation unit 33a-3, where the log-amp detectors 33a-1 and 33a-2 generate analog-voltage signals Va(Volt) and Vb(Volt) that correspond to the amplifier output signal and amplifier input signal, respectively, and inputs them to the computation unit 33a-3. The Va and Vb is represented by the following equations.Va=10Pa/20 Vb=10Pb/20  (1)Here, Pa and Pb are the powers (dB notation) that are input to the log-amp detectors 33a-1 and 33a-2.
The computation unit 33a-3 calculates the gain-detection voltage Vout(V) from the equation below.
                                                        Vout              =                                                1.2                  ×                                      log                    ⁡                                          (                                              Va                        /                        Vb                                            )                                                                      +                0.9                                                                                        =                                                1.2                  ×                                      log                    ⁡                                          (                                                                        10                                                      Pa                            /                            20                                                                          /                                                  10                                                      Pb                            /                            20                                                                                              )                                                                      +                0.9                                                                                        =                                                1.2                  ×                                      {                                                                  log                        ⁡                                                  (                                                      10                                                          Pa                              /                              20                                                                                )                                                                    -                                              log                        ⁡                                                  (                                                      10                                                          Pb                              /                              20                                                                                )                                                                                      }                                                  +                0.9                                                                                        =                                                1.2                  ×                                      (                                                                  Pa                        /                        20                                            -                                              Pb                        /                        20                                                              )                                                  +                0.9                                                                                        =                                                0.06                  ×                                      (                                          Pa                      -                      Pb                                        )                                                  +                0.9                                                                        (        2        )            From Equation (2), the power difference between the two, or in other words, the gain-monitor voltage Vout(V) that corresponds to the gain is output as shown in FIG. 15 with a slope of 0.06 V/dB, and becomes 0.9 V when the power difference is 0 dB. The attenuator 6d that is located between the directional coupler 6c and the gain-detection unit 33a on the output side is for making the input levels that are input to the two log-amp detectors 33a-1, 33a-2 the same at the rated output.
Assuming that there is power amplifier failure when the gain drops a set dB level or more (for example, 3 dB), the alarm-threshold-value-generation unit 33b outputs a voltage (=0.9−0.06×3=0.72 V) that corresponds to the gain at the point when the gain has dropped more than that set dB level as a fixed alarm threshold voltage VAL (see FIG. 15). Also, during operation, the alarm-threshold-value-generation unit 33b compares the gain-monitor voltage Vout that is calculated from Equation (2) with the alarm-threshold voltage VAL, and when the gain-monitor voltage becomes less than the alarm-threshold voltage, it determines that the power amplifier 6 has failed and outputs a gain-fluctuation alarm.
FIG. 16 shows the input amplitude level vs. gain characteristics (AM-AM characteristics) 101 and the gain-monitor voltage characteristics 102 of the power amplifier as described above, and also shows the alarm-threshold voltage VAL. As can be clearly seen from the input amplitude level vs. gain characteristics 101, the gain of the power amplifier 6 becomes a constant 50 dB for a reference input (−10 dBm) or less, and the gain-monitor voltage Vout is adjusted so that it becomes a constant 0.9 V. In the operating range, when the alarm-threshold voltage VAL is set at 0.72 V so that an alarm will be output when the gain drops from the normal gain (=50 dB) by 3 dB or more.
As described above, in FIG. 16 the gain is nearly constant in the operating range even though the input-amplitude level may vary, and the gain-monitor voltage Vout is also constant. On the other hand, as shown in FIG. 17, in recent power amplifiers 6 there is a trend for the gain to change according to the input-amplitude level in order that the amount of power consumption may be lowered. In other words, as the input-amplitude level becomes lower, there is a tendency for the gain of the power amplifier to become lower. When the gain changes in this way according to the input-amplitude level, the gain-monitor voltage Vout also changes as shown by the gain-monitor voltage characteristics 102.
Conventionally, since the gain was constant, there was no problem even though the threshold value of the gain-fluctuation alarm was set to a constant value, however, in the case of a power amplifier whose gain is not constant, even though the amplifier may be operating properly, there is a problem in that the gain-monitor voltage Vout drops as shown in FIG. 17 and becomes lower than the threshold value VAL of the gain-fluctuation alarm, and thus an alarm is output by mistake. In the example shown in FIG. 17, as the input level becomes low, the gain-monitor voltage Vout approaches the alarm-threshold voltage VAL, and when the input-amplitude level is near −35 dBm, an alarm will be generated by mistake.
Furthermore, in a power amplifier whose gain is not constant the gain-monitor voltage fluctuates according to the following two cases. In other words the gain-monitor voltage fluctuates:
(1) when the input-amplitude level vs. gain characteristics differ for each individual power amplifier, for example when there are variations in the gain slope SL (see FIG. 17); and
(2) when the gain changes due to changes in the operating environment such as temperature or power supply.
Therefore, in these cases, when the alarm-threshold voltage VAL that follows the fluctuation in the gain-monitor voltage is not set, there is a problem in that it is not possible to correctly detect amplifier failure.
There is prior art (JP2003-8360A) that detects an error state in a feedback loop based on changes of the distortion-compensation coefficient of the power amplifier per unit time, and controls the output power of the power amplifier according to an alarm signal when an error is detected. However, this prior art does not detect failure of the power amplifier. When there are changes in the gain characteristics with respect to the input-amplitude level, there is no prior art that accurately detects failure of a power amplifier.